filter.cyso.net has no CNAME records.
Question and response
Question
dig filter.cyso.net. NS
Answer
The response has no answer records.
Authority
cyso.net. 300 SOA ns1.cyso.nl. dnsadmin.cysonet.com. 1771926823 16384 2048 1209600 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0